The operation of an aerial vehicle (e.g., a manned or unmanned vehicle such as an airplane, a helicopter, a dirigible or another airship) is dependent upon a combination of four forces, namely, thrust, drag, weight and lift, the net effects of which may determine an extent and a direction of a velocity of the aerial vehicle. Thrust is a force that is typically generated by one or more aerial propulsors or propulsion units such as rotating bladed propellers or jet engines. Thrust may have a magnitude defined by one or more operating characteristics of the propulsor, e.g., a rotating speed, a number of blades, or sizes of blades of a propeller, or an amount or speed of exhaust expelled from a jet engine, as well as a direction defined by an orientation of the propulsor with respect to an airframe of an aerial vehicle. Thrust is necessary in order to overcome drag, which is a resistive aerodynamic force that is directed in an opposite direction to a direction of travel of the aerial vehicle, due to air that opposes the forward motion of the aerial vehicle. Weight is a force resulting from the Earth's gravitational pull acting on a center of mass of the aerial vehicle, in a vertical direction toward the Earth's center. Lift is another aerodynamic force that is generated by propellers, or from flows of air over wings or other control surfaces. Lift counteracts the effects of weight on an aerial vehicle, at least in part. Thrust, drag, weight and lift acting on an aerial vehicle must be placed in balance in order to ensure that the aerial vehicle operates at a desired and safe velocity.
With the exception of weight, each of the forces acting on an operating aerial vehicle may be affected by wind passing above, below or around the aerial vehicle. Wind may include a number of components that impact an amount of lift generated by a fixed or rotating wing on an aerial vehicle, as well as an extent of thrust or drag applied to the aerial vehicle. For example, a headwind is wind blowing on a front of an aerial vehicle, opposite to its direction of travel, while a tailwind is wind that blows from behind an aerial vehicle, in its direction of travel. Meanwhile, a crosswind is wind that blows laterally into an aerial vehicle, parallel to ground below the aerial vehicle and perpendicular to its direction of travel. Updrafts and downdrafts are winds that blow perpendicular to the ground and originate above or below an aerial vehicle, respectively. Wind that contacts an aerial vehicle typically includes one or more components (e.g., headwinds, tailwinds, crosswinds, updrafts or downdrafts) that impart forces on the aerial vehicle from a number of different directions.
The ability to determine air velocities is particularly advantageous for the safe operation of an aerial vehicle. The presence of air flow above, below or around an aerial vehicle may impact the aerial vehicle's ability to complete a mission or, in many cases, to remain aloft. Currently, wind speeds or directions may be determined in a number of ways. For example, a wind speed may be determined based on visual cues, such as according to the Beaufort scale, which is used to label winds with numbers (e.g., 0 to 12), descriptors such as “calm,” “gale,” or “storm force” depending on qualitative factors such as the visible effects of wind on trees, structures or bodies of water. A wind direction may be determined using a wind sock or a weathervane, which may realign itself in the presence of wind in order to minimize resistance. Additionally, an aerial vehicle may be equipped with a Pitot tube, which is an instrument that is used to determine air speeds based on static and dynamic pressures, and volumetric flow of an air stream passing through the tube. Depending on its size, an aerial vehicle may be outfitted with several Pitot tubes, which may be operated singly or in parallel to estimate air speeds in or around the aerial vehicle.
Pitot tubes are most effective at determining air speeds of an aerial vehicle when the aerial vehicle is traveling at high speeds. For example, in some jumbo jets that are configured for travel at speeds of hundreds of miles per hour, Pitot tubes may be provided in pairs and used to estimate air speeds. Pitot tubes are less effective, however, at determining air speeds when an aerial vehicle is traveling at low speeds. For this reason, Pitot tubes are not commonly used to determine speeds of wind passing above, below or around aerial vehicles that are configured to operate in a hovering flight mode, such as helicopters or many unmanned aerial vehicles (or UAVs or drones). The need to determine velocities of air and/or wind is particularly acute when aerial vehicles are operating in a hovering flight mode, as wind acting on an aerial vehicle that is traveling at low speeds or is hovering may easily upset a balance between thrust, drag, weight and lift forces acting on the aerial vehicle.